This invention relates to a distance measuring apparatus of the type which may be set on an automobile and uses electromagnetic waves such as laser light to detect the presence of a car in front and to obtain data on its position.
Development of radars (or distance measuring apparatus) to be on an automobile for monitoring the conditions in front has been continuing, including the use of laser light. These apparatus are adapted to transmit electromagnetic waves and to measure the distance to an object in front from the delay time which elapses until reflected waves are received, or to scan an area in front to determine the direction at which a target object of detection is located. If the detection sensitivity of such an apparatus for reflected waves is constant, however, the apparatus may fail to detect an object within its detection area when it is snowing or the weather condition is otherwise adverse, or depending on the surface condition of the target object of detection, such that the intensity of the reflected waves is weakened.
In view of this problem, Japanese Patent Publication Tokkai 10-197635 has disclosed an apparatus comprising means for detecting an adverse weather condition such as rain and snow and adapted to change the transmission condition of laser light such as the speed of its scanning and the frequency of light transmission to adjust the detection time (that is, the duration of time over which received signals are integrated) depending on the weather condition detected by this means such that the detection sensitivity can be improved. As another example, Japanese Patent Publication Tokkai 2000-275340 has disclosed a laser radar adapted to calculate a rate of atmospheric attenuation of reflected waves from a standard target object and to integrate the received signals for a number of times corresponding to this rate of attenuation. Both are adapted to increase the quantity of received signals to be cumulatively added by integration as the weather condition deteriorates and to thereby suppress the noise effect and to improve the sensitivity to maintain the detection capability of the radar under adverse weather conditions.
With these prior art apparatus, either a detector of weather conditions is required or the rate of atmospheric attenuation must be obtained. Thus, the physical structure of the apparatus and the control process become complicated and hence these prior art apparatus are disadvantageous from the points of view of both the cost and the space for installation. An additional disadvantage of these prior art apparatus is that they cannot be adjusted against the surface condition of the target object of detection such as when the surface is stained or dirty. Apparatus disclosed in aforementioned Japanese Patent Publication Tokkai 2000-275340 are particularly disadvantageous because they cannot be used effectively if there is no suitable standard object. In the case of radar device set on an automobile, in particular, the weather conditions are likely to change from one moment to another and a standard object is usually very difficult to set.
It is therefore an object of this invention to provide a distance measuring apparatus with a simple structure capable of preventing its sensitivity from becoming lowered due to deteriorated weather conditions or the surface condition of the target object of detection without the necessity of detecting the weather conditions.
A distant measuring apparatus embodying this invention may be characterized as comprising what are herein referred to as light-transmitting means, light-receiving means, memory means, area-combining means and judging means. The light-transmitting means is for transmitting electromagnetic waves to scan in a specified scan direction a specified detection area which is partitioned into a plurality of standard areas of a fixed width. The light-receiving means is for receiving reflection of the electromagnetic waves transmitted from the light-transmitting means and reflected by a target object which may be in the detection area. The memory means is for storing signals from the light-receiving means corresponding to individual standard areas. The area-combining means is for carrying out cumulative addition of the signals stored in the memory means corresponding to a set of those standard areas which are mutually adjacent in the scan direction. The result of the cumulative addition is defined as a combined signal associated with a single combined area which is defined as consisting of the set of standard areas if the signals corresponding to these standard areas do not exceed a specified threshold value. Those of the standard areas, the signal from which is not cumulatively added, are each defined as one combined area, the signal therefrom being defined as the combined signal associated with it. The judging means is for judging presence/absence, as well as the position, of a target object based on the combined signals from the combined areas as defined above and obtained by the area-combining means.
In the above, what is referred to as the signal to be stored in the memory means is the data that are received corresponding to each of the standard areas as a result of transmitting and receiving electromagnetic waves once or more times, such as waveform data showing the variations in intensity with time. If transmission and reception take place more than once corresponding to each standard area, the result of the cumulative addition of these more than one signals received within the corresponding standard area is the xe2x80x9csignal to be stored.xe2x80x9d The area-combining and judging means may be comprised of a microcomputer.
If a signal exceeding the threshold value cannot be obtained in a plurality of mutually adjacent standard areas, signals corresponding to them are cumulatively added by the area-combining means according to this invention and these standard areas are combined together to form a single one of what are herein referred to as the combined areas having the result of this cumulative addition as its corresponding signal. Presence or absence of a target object for detection and its position are judged on the basis of these combined areas and their corresponding signals. Thus, in a situation where reflected waves do exist from a target object and the received signals would normally exceed the target value but fail to do so because of the weather condition such as snow or rain and/or the reflecting surface condition of the target object, the aforementioned cumulative addition will be repeated and the signal intensity will increase and the actual detection sensitivity will be improved although the resolving power of detection will diminish accordingly. As a result, deterioration of detection capability of the apparatus due to weather and surface conditions can be prevented. Since environmental conditions such as the rate of atmospheric attenuation need not be separately measured, the apparatus does not become bulky or complex. If the standard areas are made sufficiently narrow, the resolving power can be improved and such an apparatus installed on an automobile may be able to detect the traffic lane in which the car in front is traveling or even the type of such a car in front.
According to a preferred embodiment of the invention, the area-combining means will operate such that, if neither the result of cumulative addition of signals corresponding to a set of mutually adjacent standard areas nor the signal from the next one of the standard areas after this set of standard areas in the scan direction exceeds the threshold value, cumulative addition will be repeated by including the signal from the next standard area and the next standard area will be included together in the single combined area corresponding to the set of standard areas. In this manner, the number of standard areas combined in a single combined area may be sequentially increased.
For example, if the signal corresponding to one standard area exceeds the threshold value, this signal is defined as the signal corresponding to one combined area (although this xe2x80x9cone combined areaxe2x80x9d consists of only one standard area). If the signal corresponding to a certain standard area does not exceed the threshold value but the signal from the next standard area in the scan direction does, these two signals are treated as signals corresponding to two different combined areas. If each of the signals corresponding to two mutually adjacent standard areas fails to exceed the threshold value, these two signals are cumulatively added and this added result is defined as the signal corresponding to a single combined area consisting of these two mutually adjacent standard areas. If this added result still fails to exceed the threshold value and the signal corresponding to the next (third) standard area also fails to exceed the threshold value, the signals corresponding to all these three standard areas are cumulatively added this added result is treated as the signal corresponding to a newly formed single combined area consisting of all these three mutually adjacent standard areas. This process is continued sequentially as long the signal corresponding to the newly created combined area fails to exceed the threshold value.
In this manner, the number of standard areas that are combined increases continuously according to the degree of adverse weather or other circumstantial conditions and hence the actual detection sensitivity varies in small steps. Thus, an optimal sensitivity level (or the lowest sensitivity level necessary for each given condition) can be attained automatically according to this invention.
In the above, a certain upper limit number may be preliminarily defined such that, if the cumulative addition is repeated and the number of standard areas combined in the cumulative addition reaches this specified upper limit number but if the signal corresponding to the combined standard areas does not exceed the threshold value, the cumulative addition process is stopped there and this upper limit number of combined standard areas is defined as one combined area. In this manner, it can be prevented to keep repeating the cumulative addition process senselessly over an unreasonably large number of times and to thereby introduce a judgment error.
According to another preferable embodiment of the invention, the judgment means is programmed so as to invalidate its own judgment result corresponding to a combined area if the size of the target object in the scan direction estimated from the width of the combined area is considered abnormally large although the signal corresponding to this combined area exceeds the threshold value, judging that the target object is not in the corresponding combined area. In this manner, only the judgment results corresponding to the combined areas having widths that may be considered reasonable (that is, not unreasonably or abnormally large) in view of the estimated size of the target object for detection, are considered valid and those corresponding to combined areas with unreasonably large widths are invalidated. In this manner, it can be avoided to erroneously conclude that the target object is present as a result of repeating the cumulative addition of signals over an unreasonably large number of standard areas.
If the transmission of the electromagnetic waves is carried out by changing the direction of transmission (that is, with a rotary motion of the light source), the detection area increases radially with the distance. Thus, the estimated size of a target object corresponding to a combined area depends not only on the width of the combined area but also on the distance to the target object.
The judgment means according to a further preferable embodiment of the invention may be programmed to validate its judgment result corresponding to a combined area in such a situation if time rate change of the estimated size and/or position of the target object obtained by the judgment means remains smaller than a specified maximum value over a specified length of time, although the judgment result will be invalidated if otherwise. This embodiment is preferable because even a distant target object with poor reflectivity can be correctly detected although it may be detected as having an unreasonably large width. In other words, there are situations where a proper target object may appear to the measuring apparatus as having an unreasonably large width and the invention can serve to detect such a target object correctly by identifying it as a target object.
To explain such situations more in detail, the beam profile of the transmitted electromagnetic waves is usually a Gaussian curve. If the standard areas are set so as to have a fixed width of 1xc2x0, this width is set so as to include the parts of the curve with intensity greater than 50% of its peak value. In other words, the area width represents a range with sensitivity above a certain level and there is some degree of sensitivity outside this range. Thus, reflected waves from a target object are also being received to a certain extent also in outside areas and although there is only one target object, its width may be estimated also from these outside areas. Let us assume that the target object is an automobile in front at a distance of 100 m. If the combined area including such outside areas subtends an angle of 3xc2x0, the estimated width of the target object will be 5.2 m (100 mxc3x97tan 3xc2x0) which is much greater than the width of an ordinary automobile (that is, 1.5 m-2.5 m). In other words, there are probably many situations where the estimated width of the target object may be unreasonably large but it actually represents the width of one automobile.
According to this embodiment of the invention, misjudgments in such situations can be avoided. If a target object appears to have an unreasonably large width because this target object actually represents two automobiles or a combination of an automobile and a reflecting roadside object, they will be separated soon and they will quickly cease to be detected as one combined target object. If the target object is a single automobile although appearing to have an unreasonably large width, however, it will continue to appear as one single object over a significant length of time. In the above, xe2x80x9ctime rate of changexe2x80x9d may mean, if measurements are taken periodically, the difference between results of measurements at one time and at the next time.
In any of the embodiments of the invention described above, the threshold value may be set according to a time average of signals on which the judging means makes a detection judgment, or according to signals received while the electromagnetic waves are not being transmitted. In this manner, the threshold value changes according to an average signal value or the noise level, and this can significantly decrease the possibilities of a detection error due to noise under adverse environmental conditions or an increase of the noise component due to an increased number of times the cumulative addition is repeated. The threshold value may be made time-dependent and to decrease as the delay time until the electromagnetic waves are received by the light-receiving means increases. In other manner, the threshold value is set to be relatively large for signals corresponding to a relatively shorter distance away and the problem of detection error due to weather conditions can be eliminated.
The distance measuring apparatus of the invention may be structured such that the scanning is performed only by the light-transmitting means. In order to improve the distance-measuring capability, however, it is preferable to operate a receiver head comprising the light-receiving means in synchronism with the light-transmitting means for the scanning.